Ukraine is a sovereign state independent from russia. russia dreams about soviet union comeback. Some countries are out of reach now, e.g. Baltic states which became EU and NATO members.
After russians being assholes to their neighbours for decades (if not centuries) - some neighbours decided that it's time to leave abusive relationship.
To russia Ukraine is integral only for couple of things - land buffer from the west influence, land bridge to the black sea (Crimea).
Ukrainians see how russia abuses them (e.g. frequent embargoes or price jumps for gas, and other tightly coupled economical relations they are not allowed to escape) and attempts to leave this. That's a deep blow to russia and their maniac dictator cannot allow this. F
For ages - instead of russia attempting to build proper strong and attractive economy are using force on their small neighbours. I think every single neighbor of russia (or russian empire or soviet union) was either at war or occupied in previous 100 or 200 years. Neighbouring nations (the people, not the politicians or greedy oligarchs) are attempting to flee russian influence zone, due to constant economic pressure and imminent danger. russia now attempts to paint this as "NATO expansion" or "US imperialism", but they are just old-school bully.